1. Field of the Invention
This invention relates to a method for minimizing hydrogen halide corrosion in a partial oxidation, entrained flow, quench gasifier.
2. Description of the Prior Art
Petroleum, coal and other organic natural resources are used as fuels, for transportation, heating and power generation, and also as feedstocks to partial oxidation gasifiers for the manufacture of various industrial chemicals. This includes gaseous, liquid and solid hydrocarbons. Almost any combustible material having carbon and hydrogen can serve as a feedstock for the partial oxidation gasification process, such as natural gas, methane, crude oil, shale oil, bitumen, heavy residual oil, coal, petroleum coke, sewage sludge, light hydrocarbon fuel, various coals including anthracite, bituminous, sub-bituminous and lignite, and various mixtures of the above. Also useful as feedstocks for the partial oxidation gasification process are various mixtures and slurries of the above, using among other things, hydrocarbons or other gaseous or liquid materials to form a pumpable slurry.
Diminishing natural resources as well as economic considerations have led to the increasing use of organic feedstocks from impure sources, such as scrap or waste plastic materials which contain relatively high levels of contaminants.
The scrap plastic is used as a hydrocarbonaceous feedstock in a partial oxidation reaction to produce mixtures of hydrogen and carbon monoxide, referred to as synthesis gas, or simply "syngas." Syngas can be used to make other useful organic compounds or as a fuel to produce power.
Waste or scrap plastic materials often comprise at least one solid carbonaceous thermoplastic and/or thermosetting material which may or may not contain associated inorganic matter, such as fillers and reinforcement material. Such materials may be derived from obsolete equipment, household containers, packaging, industrial sources, recycling centers and discarded automobiles. Scrap plastic comprises solid organic polymers derived from sheets, films, extruded shapes, moldings, reinforced plastics, laminates and foamed plastics. The mixture of scrap plastics varies with the source and with the presence of non-combustible inorganic matter incorporated in the plastic as fillers, catalysts, pigments and reinforcing agents.
Inorganic matter may also include dyes and pigments such as compounds of cadmium, chromium, cobalt and copper; nonferrous metals such as aluminum and copper in plastic coated wire cuttings; metal films; woven and nonwoven fiber glass, graphite, and boron reinforcing agents; steel, brass, and nickel metal inserts; and lead compounds from plastic automotive batteries. Heavy metals, for example cadmium, arsenic, barium, chromium, selenium, and mercury may also be present. The inorganic constituents may be present in the solid hydrocarbonaceous plastic-containing material in an amount ranging from about a trace to about 30 weight percent of such plastic-containing material.
Solid hydrocarbonaceous plastic waste typically consists of polyethylene, polyethylene terephthalate, polypropylene, polyesters, polyurethanes, polyamides, polyvinylchloride, polystyrene, cellulose acetate and mixtures thereof. Also found are polyurea, polycarbonates, cellulose, acrylonitrile-butadiene-styrene (ABS), acrylics, alkyds, epoxy resins, nylon, phenolic plastics, polyacetals, polyphenylene based alloys, styrene, acrylonitrile, thermoplastic elastomers, fluoride polymers, rubber stocks, urea and melamine.
Because of the reducing conditions which exist during the partial oxidation reaction, the halogen content of the halogenated organic material is converted to hydrogen halide or ammonium halide. Free halogens which can be formed during the complete combustion of halogenated organic materials are not formed under the conditions in the partial oxidation gas reactor. Other halogen compounds such as phosgene (COCl.sub.2), cyanogen chloride (CNCl), volatile halide compounds such as AlCl.sub.3, Cl.sub.4 and POCl.sub.3 are not formed during the partial oxidation reaction even though aluminum, vanadium or phosphorus may be present in the feedstock.
A quench gasifier is used to conduct partial oxidation reactions and to capture most of the acid component of the syngas in the quench water. The partial oxidation reaction is carried out in a free-flow unpacked noncatalytic quench gasifier. The reaction temperature is about 1800.degree. F. to about 3000.degree. F. and the reaction pressure is about 1 to about 100 atmospheres, preferably about 25 to about 80 atmospheres. Under such high temperatures and pressures, substantially all halogenated organic materials are rapidly converted into hydrogen halides, carbon dioxide, carbon monoxide, hydrogen cyanide, ammonia, carbonyl sulfide, hydrogen and trace amounts of other gases and small quantities of carbon.
The presence of halogens in scrap plastic may range from trace amounts to as much as 10 weight % or more, and can cause severe corrosion problems. During the non-catalytic partial oxidation reactions, the halogen content of the scrap plastic hydrocarbonaceous feeds is primarily converted to hydrogen halides. At high temperatures, these acidic halides make heat recovery from the resultant syngas impractical because of corrosion problems prior to removal of the acidic component. This problem is compounded when the halogen content is highly variable and cannot be easily measured, as is the case with any waste material, particularly plastics, thereby making implementation of corrective countermeasures very difficult.
The extent of corrosion in the quench portion of the reactor and in downstream equipment can be reduced by adding a base to control the pH. A second problem arises from the variable nature of the halide content of the feed. If the pH in the water system becomes too high, depending on the base used, salts can precipitate in parts of the process water system causing blockage and/or obstruction. It is difficult to control the addition rate of base to match the halide feed rate because pH control of hot, pressurized process water or feedforward control based on halide analysis of the feed is unreliable.
In the gasification of well defined feedstocks containing low amounts of halides, on the order of about 0.05 weight % to about 2 weight %, the addition of a base to neutralize the acidic halide content of gasification water streams is based on stoichiometric amounts of the base for the average amount of acid produced in the gasifier. Since the buffering capacity of the process water is typically greater than variations in the acid generating components in the feed, the pH of the process water can be measured offline and addition rates of base adjusted as needed. However, when the amount of halides in the feed can far exceed the natural buffering capacity of the process water, and where this amount can change rapidly, such a system cannot prevent severe corrosion. This can occur when there is at least about 0.5 weight % halide in the feed, and normal or low amounts of nitrogen or alkali metals in the feed, where "normal" is defined as those amounts typically found in oil residuum, coal or petroleum coke.
U.S. Pat. No. 4,468,376 to Suggitt discloses a method for disposing of fixed amounts of halogenated organic material produced during a partial oxidation reaction. For example, the quench zone of the gasifier is maintained at a pH above 7 with ammonia to ensure that there is a sufficient amount of ammonia to react with the hydrogen halide in the synthesis gas stream. Thus, the Suggitt patent discloses how to prevent contamination of synthesis gas with acid, but does not disclose how to prevent corrosion in the process water system. The Suggitt patent does not explain how to control ammonia addition when the feedstock to the gasifier contains variable amounts of halide, which is the situation encountered when using waste plastic materials as the feedstock.
If an excessive amount of ammonia or other equivalent base is added to the quench water so that the pH rises above about 10, solid ammonium carbonate salts will precipitate as a result of the reaction of NH.sub.3 or the cationic portion of another equivalent base with dissolved carbon dioxide in any water contacted with or condensed from the syngas. This can cause plugging problems in the water system used in the gasification operation, particularly the heat exchangers used to cool the syngas from the quench and scrubber temperatures of about 350.degree. F. to about 600.degree. F., to temperatures of about 80.degree. F. to about 300.degree. F., for further cleanup or processing.
Alternatively, if the amount of ammonia or other equivalent base is insufficient to react with all the hydrogen halide in the synthesis gas stream, rapid and catastrophic acid corrosion of materials of construction, such as carbon steel, can occur.
Furthermore, the presence of ammonia or any volatile base contaminates the synthesis gas product. Therefore the process has to be adapted to remove the neutralizing base from the synthesis gas and to recover or dispose of any excess base.
If one skilled in the art followed the teachings of the Suggitt patent, it could lead to large amounts of excess water or plugging and blockage in parts of the water system used in the partial oxidation reaction system. The Suggitt patent also does not disclose specific means to assure that ammonia is always in excess, particularly for those feeds where the halide content varies widely. Thus, the Suggitt patent does not disclose how pH control with ammonia can be implemented, or how to deal with feedstocks containing a variable halogen content. In essence, the Suggitt patent does not disclose how to control pH or corrosion as the halide content of the feed varies, nor does it address the problem of ammonia contamination in the synthesis gas.
When gasifying a feed with high and variable chloride content on the order of about 10 weight % to about 15 weight %, where ammonia addition is based on periodic off-line sample analyses, for example, every 30 minutes, a sudden increase in the chloride content of the feed caused the quench water pH to drop below 1 in less than 15 minutes. Failure was experienced in a carbon steel component of the process water piping due to corrosion which caused a hole in the line. The time required for the pH to decrease to dangerous levels, if ammonia addition is interrupted or inadequate, can be less than 30 minutes in a gasifier with more than 2 weight % chloride in the feed. This can cause rapid corrosion of any carbon steel component, and can result in the release of pressurized hot gas or water.
In similar circumstances to those cited above, difficulties in adjusting the output of the ammonia addition pump caused the ammonia addition rate to be roughly twice the required amount for neutralization. This caused rapid plugging of the synthesis coolers in less than one hour, resulting in shutdown of the reactors.